Distribution of Uranium and Thorium Isotopes in Colloidal and Dissolved Fraction of Water from San Marcos Dam, Chihuahua, Mexico

In the NW of Chihuahua City, Mexico, San Marcos range formation is located. San Marcos area includes two uranium mineral outcrops, a river and its dam, which are the main water supply for the nearest agricultural lands. Assessing fractionation and distribution of U and Th isotopes contained in colloidal and dis-solved fractions of surface water is the aim of this work. U and Th isotopes’ activities were obtained by alpha spectrometry. 238U and 232Th contents in colloids ranged from 5 to 16, and from 0.3 to 0.5 mBq·g

−1

, respectively, with activity ratio (AR, 238U/234U) of up 6. The U distribution coefficient (kdU) ranged from 1·10

3

to 7·10

3

L·g

−1

. The results have shown a possible lixiviation of uranium from geo-logical substrate into the surface water and an important fractionation of uranium and thorium.